Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.
ECG rhythm strip interpretation
Paper (25 mm/s, 10 mm/mV)
1 small box = 0.04 s, 1 large box = 0.20 s, 10 mm = 1 mV
Heart rate, regular rhythm
300 ÷ large boxes = 1500 ÷ small boxes = 60 ÷ RR (s)
Heart rate, irregular rhythm
QRS complexes in 6 s × 10
Normal PR interval
0.12–0.20 s
Normal QRS duration
< 0.12 s
QTc (Bazett)
QT/√RR
QTc (Fridericia)
QT/∛RR
QTc (Framingham)
QT + 154·(1 − RR)
Sinus rates
bradycardia < 60, normal 60–100, tachycardia > 100 /min
Escape rates
junctional 40–60, ventricular 20–40 /min
Fast rhythms
SVT 150–250 /min (narrow), VT > 100 /min (wide)
Method
Rate: for a regular rhythm divide 300 by the large boxes or 1500 by the small boxes between two R waves; for an irregular rhythm count the QRS complexes in 6 seconds and multiply by 10.
Rhythm: compare the R–R intervals. Equal is regular; a repeating pattern is regularly irregular; no pattern at all is irregularly irregular.
P waves: is there an upright P wave in lead II before every QRS, all alike? Look for fibrillatory or sawtooth waves, inverted P waves and P waves without a QRS.
PR interval: from the start of P to the start of the QRS, normally 0.12–0.20 s and constant. Watch whether it lengthens, stays fixed before a dropped beat, or varies at random.
QRS duration: below 0.12 s is narrow; 0.12 s or more is wide. Then name the rhythm, and for the QT use QTc = QT/√RR (Bazett) with times in seconds.
Evidence-based practice statistics
Sensitivity and specificity
Se = a/(a + c), Sp = d/(b + d)
Predictive values
PPV = a/(a + b), NPV = d/(c + d)
Accuracy
(a + d)/n
Likelihood ratios
LR+ = Se/(1 − Sp), LR− = (1 − Se)/Sp
PPV at prevalence p
Se·p / (Se·p + (1 − Sp)(1 − p))
Odds
odds = p/(1 − p); p = odds/(1 + odds); post-test odds = pre-test odds × LR
Risk measures
RR = EER/CER, ARR = CER − EER, RRR = ARR/CER
Number needed to treat
NNT = 1/ARR, rounded up; NNH = 1/(EER − CER)
Odds ratio
OR = ad/(bc)
Incidence rate
new cases / person-time
Method
Draw the 2 × 2 table and label a, b, c and d before you calculate.
Read sensitivity and specificity down the columns, predictive values along the rows.
For another prevalence, imagine 10 000 people and fill the table with Se, Sp and the prevalence.
For a likelihood ratio, convert the probability to odds, multiply, and convert back.
For a trial, find EER and CER first; then RR = EER/CER, ARR = CER − EER and NNT = 1/ARR rounded up.
2Practice problems
1.Human anatomyMedium
Which cranial nerve supplies the trapezius and sternocleidomastoid muscles?
AHypoglossal nerve (XII)
BAccessory nerve (XI)
CVagus nerve (X)
DFacial nerve (VII)
2.Human anatomyHard
In the femoral triangle, what is the order of the main structures from lateral to medial?
AFemoral nerve, femoral artery, femoral vein
BFemoral nerve, femoral vein, femoral artery
CFemoral vein, femoral artery, femoral nerve
DFemoral artery, femoral nerve, femoral vein
3.Human physiologyMedium
During contraction of a skeletal muscle sarcomere, which band stays the same length?
AI band
BH zone
CThe whole sarcomere
DA band
4.Human physiologyHard
A young woman has muscle weakness that gets worse with repeated use and improves with an acetylcholinesterase inhibitor. What is the most likely mechanism?
ADemyelination of central nervous system tracts
BAntibodies against acetylcholine receptors at the neuromuscular junction
CLoss of motor neurons in the spinal cord
DA defect in the dystrophin protein of muscle
5.MicrobiologyMedium
Which antibody class is the first to rise in a new infection?
AIgA
BIgG
CIgM
DIgE
6.MicrobiologyHard
How do high-risk HPV types contribute to cervical cancer?
AThe virus destroys CD4 T cells in the cervix
BViral E6 and E7 proteins inactivate the tumour suppressors p53 and Rb
CThe virus produces an endotoxin that damages DNA
DThe virus kills cells so quickly that they are replaced by cancer cells
7.PharmacologyMedium
What should the nurse teach a patient about taking levothyroxine?
ATake it together with the iron tablet to save time
BTake it only on days when feeling tired
CTake it on an empty stomach, 30–60 minutes before breakfast
DTake it with milk at dinner, as calcium protects the stomach lining
8.PharmacologyHard
Ten days after chemotherapy, a patient has a temperature of 38.5 °C (101.3 °F) but says she feels fine. What should happen?
AWait 48 hours to see whether the fever settles
BUrgent cultures and IV broad-spectrum antibiotics without delay
CStart oral antibiotics only if a rash develops
DParacetamol at home and review at the next routine clinic visit
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.20, PaCO₂ 63 mmHg (8.4 kPa) and HCO₃⁻ 24 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.20PaCO₂ 63 mmHg (8.4 kPa)HCO₃⁻ 24 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
A practice gas shows pH 7.33, PaCO₂ 33 mmHg (4.4 kPa) and HCO₃⁻ 17 mmol/L, with Na⁺ 138 and Cl⁻ 87 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.
This lead II strip shows atrial flutter. How many flutter waves per QRS complex are there (the n of n:1 conduction)?
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
flutter waves per QRS =
12.ECG rhythm strip interpretationHard
Interpret this lead II strip with the five steps in order: rate, rhythm, P waves, PR interval, QRS duration. At which step do you find the first abnormal finding? Click it.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
1rate
2rhythm
3P waves
4PR interval
5QRS duration
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a cohort, groups of people were followed for different lengths of time (see the table: number of people and years of follow-up for each). During follow-up, 4 new cases occurred. Find the incidence rate IR per 1000 person-years. Round to one decimal place.
80 × 0.5 years260 × 1 year4 new cases
IR = per 1000 person-years
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 93% and a specificity of 93%. It is used in a population where the prevalence is 15%. Find the positive predictive value PPV as a percentage. Round to one decimal place.
Se = 93%Sp = 93%p = 15%
PPV = %
3Solutions
1.Human anatomy
Answer: B · Accessory nerve (XI)
Why this is the answer
The accessory nerve (XI) supplies sternocleidomastoid and trapezius. Damage weakens head turning and shoulder shrugging.
Answer
B · Accessory nerve (XI)
2.Human anatomy
Answer: A · Femoral nerve, femoral artery, femoral vein
Why this is the answer
From lateral to medial: nerve, artery, vein, then empty space and lymphatics (NAVEL). This is why the femoral artery pulse lies between the nerve and vein.
Answer
A · Femoral nerve, femoral artery, femoral vein
3.Human physiology
Answer: D · A band
Why this is the answer
In the sliding filament model, filaments slide past each other without shortening. The A band (myosin length) stays the same while the I band and H zone shorten.
Answer
D · A band
4.Human physiology
Answer: B · Antibodies against acetylcholine receptors at the neuromuscular junction
Why this is the answer
In myasthenia gravis, autoantibodies reduce the number of functional acetylcholine receptors. Blocking acetylcholinesterase leaves more ACh to bind the remaining receptors.
Answer
B · Antibodies against acetylcholine receptors at the neuromuscular junction
5.Microbiology
Answer: C · IgM
Why this is the answer
IgM appears first in a primary response; IgG follows and provides long-term protection. A positive IgM therefore suggests recent infection.
Answer
C · IgM
6.Microbiology
Answer: B · Viral E6 and E7 proteins inactivate the tumour suppressors p53 and Rb
Why this is the answer
E6 promotes the breakdown of p53 and E7 binds Rb, removing controls on cell division and DNA repair. Infected cells can then accumulate mutations and become cancerous over years.
Answer
B · Viral E6 and E7 proteins inactivate the tumour suppressors p53 and Rb
7.Pharmacology
Answer: C · Take it on an empty stomach, 30–60 minutes before breakfast
Why this is the answer
Levothyroxine is best absorbed on an empty stomach, and calcium, iron and antacids should be taken some hours apart. It is taken every day, usually for life.
Answer
C · Take it on an empty stomach, 30–60 minutes before breakfast
8.Pharmacology
Answer: B · Urgent cultures and IV broad-spectrum antibiotics without delay
Why this is the answer
Fever during neutropenia is treated as a medical emergency because infection can progress rapidly. Cultures are taken and empirical IV antibiotics are started promptly, ideally within one hour.
Answer
B · Urgent cultures and IV broad-spectrum antibiotics without delay
9.Arterial blood gas interpretation
Answer: uncompensated
Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.
ROME, value by value
pH 7.20: acidaemia (below 7.35)
PaCO₂ 63 mmHg: high (35–45)
HCO₃⁻ 24 mmol/L: normal (22–26)
Compensation
uncompensated
Answer
uncompensated
10.Arterial blood gas interpretation
Answer: added metabolic alkalosis
Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.
Anion gap
AG = 138 − (87 + 17) = 34
Delta ratio
(34 − 12)/(24 − 17) = 3.143
added metabolic alkalosis
Answer
added metabolic alkalosis
11.ECG rhythm strip interpretation
Answer: flutter waves per QRS = 3
Rhythm criteria (lead II): sinus rhythm has an upright P wave before every QRS, a constant PR of 0.12–0.20 s and a narrow QRS: normal sinus rhythm at 60–100/min, sinus bradycardia below 60, sinus tachycardia above 100. Atrial fibrillation: no P waves, a fibrillatory baseline and an irregularly irregular R–R. Atrial flutter: sawtooth flutter waves at about 250–350/min with a fixed conduction ratio such as 2:1 or 4:1. First-degree AV block: sinus rhythm with a constant PR above 0.20 s. Junctional rhythm: narrow QRS at 40–60/min with absent or inverted P waves (PR below 0.12 s, or P after the QRS). A PVC is an early, wide (0.12 s or more), bizarre QRS without a P wave, with the T wave in the opposite direction and usually a full compensatory pause; in bigeminy every second beat is a PVC.
Flutter waves
F–F = 6 small boxes = 0.24 s → 1500 ÷ 6 = 250 per minute
QRS complexes
R–R = 18 small boxes = 0.72 s → 1500 ÷ 18 = 83 per minute
conduction ratio = 250 ÷ 83 = 3:1
Answer
flutter waves per QRS = 3
12.ECG rhythm strip interpretation
Answer: rate
Five steps for every strip: 1. rate (300 or 1500 method, or the 6-second method if irregular); 2. rhythm: are the R–R intervals equal; 3. P waves: present, upright in lead II, one before every QRS and all alike; 4. PR interval: 0.12–0.20 s and constant; 5. QRS duration: below 0.12 s. Only after the five steps name the rhythm. Normal sinus rhythm passes all five: 60–100/min, regular, upright P before every QRS, PR 0.12–0.20 s, QRS below 0.12 s.
Five steps
1. Rate: R–R = 13 small boxes, 1500 ÷ 13 ≈ 115 per minute.
2. Rhythm: regular, every R–R is 0.52 s.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.12 s, constant, within 0.12–0.20 s.
5. QRS: 0.08 s, narrow (below 0.12 s).
First abnormal step
rate → sinus tachycardia
Answer
rate
13.Evidence-based practice statistics
Answer: IR = 13.3 per 1000 person-years
Likelihood ratios and risk measures: LR+ = sensitivity/(1 − specificity), LR− = (1 − sensitivity)/specificity. In a trial, EER and CER are the event rates with and without treatment: RR = EER/CER, ARR = CER − EER, RRR = ARR/CER and NNT = 1/ARR, rounded up. In a case-control study only the odds ratio OR = ad/(bc) can be used. An incidence rate = new cases/person-time.
Person-years
PY = 80·0.5 + 260·1 = 300
Incidence rate
IR = 4/300·1000 = 13.3 per 1000 person-years
Answer
IR = 13.3 per 1000 person-years
14.Evidence-based practice statistics
Answer: PPV = 70.1 %
Predictive values depend on prevalence: PPV = Se·p/(Se·p + (1 − Sp)(1 − p)) and NPV = Sp(1 − p)/(Sp(1 − p) + (1 − Se)p). With likelihood ratios: pre-test odds = p/(1 − p), post-test odds = pre-test odds × LR, probability = odds/(1 + odds). For a population of N, true positives = N·p·Se and false positives = N·(1 − p)·(1 − Sp). NNT = 1/(CER × RRR), rounded up.
True and false positives (per person)
Se·p = 0.93·0.15 = 0.1395
(1 − Sp)·(1 − p) = 0.07·0.85 = 0.0595
Positive predictive value
PPV = 0.1395/(0.1395 + 0.0595) = 0.701 ≈ 70.1%
Answer
PPV = 70.1%
The lower the prevalence, the lower the PPV of the same test.